SUBSURFACE STRATIGRAPHIC FRAMEWORK FOR THE MANCOS SHALE OF THE PICEANCE BASIN, COLORADO

A detailed subsurface stratigraphic framework for the Upper Cretaceous Mancos Shale in the Piceance Basin, Colorado, was constructed from analysis of over 1,800 geophysical well logs. This thick, shale-dominated succession, deposited in the distal portion of the Western Interior Seaway, ranges from 1,500 feet thick in the southeast to over 5,000 feet in the northwest. The Mancos Formation is divided into four informal subsurface units, described below, with their corresponding ages and surficial “member” equivalents mapped by the Colorado Geological Survey.

The Lower Mancos (Cenomanian-Turonian) consists of 200-400 feet of mudrocks, bentonites, and thin sandstones (Graneros, Hartland, Bridge Creek, Fairport, Blue Hill, Juana Lopez, Montezuma Valley members). The 700-1,700 feet thick Middle Mancos (Coniacian-Santonian) comprises interbedded calcareous shales and shaley limestones equivalent to the Niobrara Formation (Fort Hays and Smoky Hill members). The Upper Mancos (lower-middle Campanian) consists of 700-1,500+ feet of interlaminated fine-grained sandstones and silty shales (Prairie Canyon) and bentonitic shale (Sharon Springs). The Uppermost Mancos (middle-upper Campanian) consists of eight shale intervals intertonguing with eight sandstones (Morapos, Castlegate, Loyd, lower Sego, upper Sego, Corcoran, Cozette, and Rollins/Trout Creek Sandstones). The Rollins marks the end of the Mancos seaway.

This Colorado subsurface stratigraphy aligns with the outcrop nomenclature in Utah on this conference’s pre-meeting field trip "Revisiting the Cretaceous Mancos Group in Utah: Problems, Previous Methods, and New Perspectives on a World-Class Cretaceous Marine Section" of Kirkland, King, Lively, & Bylund (2025) as follows: Lower Mancos correlating to both the "Greenhorn Cyclothem" (Jessie Twist member, Coon Springs Sandstone, Blue Hill Shale) and "Sage Breaks hemicyclothem" (Juana Lopez Formation, and Montezuma Valley Member); Middle and Upper Mancos "Niobrara Cyclothem" (Smoky Jill Member, Prairie Canyon Member); and Uppermost Mancos "Claggett Cyclothem" (Bar X Shale, lower Castlegate Sandstone, Buck Shale, Sego Sandstone, Neslen Formation).

These differences in nomenclature usage reflect lateral facies’ changes and evolving interpretations of the Mancos, driven by ongoing outcrop, subsurface, and paleontological investigations.